FIG. 1 (Prior Art) is a cross-sectional diagram of a connector 1 that is to be surface mount soldered to a printed circuit board 2. The connector 1 includes a connector body portion 3 and a set of metal pins. The end of each metal pin is referred to as a “solder tail”. Solder tail 4, for example, is the end of metal pin 5. In order to surface mount connector 1 to printed circuit board 2, solder-wettable contact pads or traces on the upper surface of printed circuit board 2 are covered with amounts of solder paste 6. The contact pads or traces are not illustrated in FIG. 1. The amounts of solder paste may, for example, be approximately 0.18 millimeters thick. Connector 1 is then placed down onto the printed circuit board 2 so that the solder tails of connector 1 are forced down into the solder paste.
FIG. 2 (Prior Art) is a cross-sectional diagram that shows connector 1 in place on the printed circuit board 2. An amount of solder paste is in contact with the bottom of each solder tail and the metal contact pad or metal trace below it. The structure of FIG. 2 is then heated in a reflow oven such that the solder in the solder paste melts and solders the solder tails to the underlying contacts pads or traces. The various parts such as the bottom of the connector and the upper surface of the printed circuit board are not, however, perfectly planar.
FIG. 3 (Prior Art) is a cross-sectional view of connector 1 being placed down onto a printed circuit board 7 that exhibits warpage. The upper surface of printed circuit board 7 is bowed. When connector 1 is placed down onto the printed circuit board prior to reflow soldering, the solder tails in the center of the connector make contact with the solder paste first. These solder tails are pushed down into the solder paste more than are the solder tails located toward the ends of the connector. The solder tails are therefore pressed down into the solder paste to different degrees. A solder tail may be pressed down into the solder paste so far that the solder paste under it is forced out from under the solder tail and to the sides of the solder tail to such a degree that the displaced solder paste bridges with the solder paste of a neighboring solder tail.
FIG. 4 (Prior Art) is an expanded view of a portion 8 of the structure of FIG. 3. Note that the amounts of solder paste under solder tails 9-12 have been forced together to form a single bridge 13 of solder paste. When the assembly of FIG. 3 is reflow soldered and is then cooled, the result can be bridges of solid solder that short adjacent pins together. This can occur even though areas of solder mask may separate adjacent contact pads under the adjacent solder tails. The solder may bridge over the solder mask.
FIG. 5 (Prior Art) is a cross-sectional diagram that illustrates connector 1 being placed down onto a printed circuit board 14 that is warped in a different way to have a concave upper surface. The solder tails in the center of the connector may be as much as 0.27 millimeters away from the upper surface of solder paste if the connector is two inches in length, if the solder tail coplanarity (alignment of the bottoms of the solder tails) is 0.2 millimeters over a distance of two inches, if the amounts of solder paste under the solder tails prior to reflow soldering is approximately 0.18 millimeters thick, and if warpage of the printed circuit board is approximately 0.250 millimeters over a distance of two inches. In this same situation, the solder tails at the ends of connector 1 are forced down into the solder paste more than the solder tails in the center portion of the connector. The same bridging problem explained above in connection with FIG. 3 may therefore occur in the structure of FIG. 5 between solder tails at the ends of the connector. A solution is desired.